Flowserve PMV P4 - Manual (Page 8)

8
The P4 and P5/EP5 operates on a force balance principal. Force is originated by the signal
pressure transmitted through a diaphragm on to the balance arm. The opposing force is
achieved through the feedback spring and is proportional to the position of the lower arm.
The lower arm position is determined by the position of the cam which is secured to the
spindle and connected to the actuator shaft thus providing the feedback from the actuator/
valve. When these two forces are equal, the balance arm and the spool in the pilot valve are
in a neutral position — the complete unit is in a balanced position. Air is supplied to the
pilot valve through port S, and controls the air flow through ports C1 and C2
Assume an equilibrium position.
An increased control pressure will deflect the diaphragm 1 down, compressing the feedback
spring 3. The balance arm 2 moves the spool 7 in the pilot valve 8 furnishing supply air to
the actuator, while at the same time air is exhausted from actuator and is vented to atmosphere
through the pilot valve and the OUT port . With the increased supply air, the actuator rotates
(or moves linearly) moving the positioner spindle 6. The spindle and cam 5 rotate, forcing
the lower arm 4 upwards compressing the feedback spring 3. This motion will continue
until the two forces are equal and the unit is in an equilibrium position.
3. Function
I 7 3 1 2
S
8 5 6 4